The development and function of the minor subset of CD4-8- double negative (DN) TCRalphabeta T cells and their relationship to the mainstream CD4+ and CD8+ T cells have been the subject of much intense research. We described recently a subset of mature CD4+ mouse thymocytes that is very similar to the DN T cells. It shares the membrane expression of the activation markers CD44 and Ly6C and the natural killer cell marker NK1.1. This population also shares the unique potential to immediately secrete a large set of lymphokines typical of Th0-activated T cells such as interleukin-4 (IL-4), IL-5, and interferon gamma (IFN- gamma). Thus, its physiologic role may be to influence the Th1/Th2 differentiation of an immune response early after the introduction of antigen. In the past year, we have focused on the development of this T cell subset and characterized its T cell receptor repertoire. The selection of these cells in the thymus requires MHC class I expression and not MHC class II, CD4, or CD8 molecules. The selecting element appears to be a nonclassical MHC class Ib molecule expressed on bone marrow-derived cells but not on thymic stromal cells. The selected T cells express a very defined set of receptors consisting of a single V alpha chain, Valpha14 J281, and three V betas: Vbeta8, Vbeta7, and Vbeta2. A similar receptor family was found in human double negative T cells. These results suggest that this special subset is uniquely selected to recognize a narrow set of evolutionarily conserved ligands.